In general, a metal vapor discharge lamp, such as a metal halide lamp or a high pressure sodium lamp, has the advantage of an excellent luminous efficacy. However, such a metal vapor discharge lamp requires a high voltage to start its operation.
In recent years, a metal vapor discharge lamp which has compatibility with a high pressure mercury vapor lamp has been developed. The compatible metal vapor discharge lamp is started using a stabilizer for mercury vapor discharge lamps. With such a metal vapor discharge lamp, however, it is necessary to lower the starting voltage sufficiently near the starting voltage of high pressure mercury vapor lamps.
An example of a technique for reducing the starting voltage is disclosed in the Japanese Patent, Sho. 60-34220 which is owned by the same applicant. The Japanese Patent discloses a metal vapor discharge lamp which contains a radiation source including radioactive material with a half-life of 0.5 to 10 years for reducing the starting voltage. In the metal vapor discharge lamp, the radiation source usually emits rays. The rays ionize metal vapor in the metal vapor discharge lamp so that electrons are generated in the metal vapor discharge lamp. The electrons resulting from the ionization may cause an initial discharge in the lamp at the starting of the metal vapor discharge lamp. Thus, the rays emitted from the radiation source operate as a seed for the initial discharge.
However, such radiation sources are likely to exert adverse influence on the human body. Therefore, careful attention must be payed to handling the radiation sources during manufacture of such a metal vapor discharge lamp containing the radiation sources.
Another example of atechnique for reducing the starting voltage is disclosed in U.S. Pat. No. 4,445,067, which is assigned to the same applicant and corresponds to Japanese Patent, Sho. 60-34222. U.S. Pat. No. 4,445,067 also discloses a metal vapor discharge lamp which contains a radiation source. The radiation source is comprised of a radioactive material and a ceramic container made of ceramic material. The radioactive material is dispersed in the ceramic container. In the metal vapor discharge lamp, the radiation source further has a production shell for sealing the ceramic container. The protection shell is made of non-radioactive, heat-resistant and corrosion-resistant material, for example, glass.
However, the metal vapor discharge lamp described in U.S. Pat. No. 4,445,067 had drawbacks as follows. For example, the radiation source requires a process for sealing the ceramic container by glass during manufacture of the radiation source. The protection shell is apt to vary in thickness. Thus, there is a risk of failure of the shell, depending on the thickness. When the protection shell, such as a glass shell, is made thicker to increase its safety, the protection shell interferes with the transmission of the rays emitted from the radioactive material therethrough. As a result, the benefit of the radiation source for the rapid starting of the metal vapor discharge lamp which is the inherent object of the radiation source, decreases.